The present invention relates to microstrip antennas with parasitic elements.
The prior art describes several design parameters for microstrip antennas. Surface waves are excited whenever a microstrip antenna has a substrate with the relative dielectric constant is greater than 1 (∈R>1). Since many preferred substrates for microstrip antennas have relative dielectric constants that range from about 2.5 (for PFTE) to 25 or higher, the problem of surface waves is one that must be mitigated or, rarely, eliminated. Surface waves interfere with desired antenna gain, bandwidth, and cross-polarization levels for microstrip antennas.
The stability of a phase center for a microstrip antenna is a critical design parameter for precision measurement GPS devices (cm or mm level accuracy) made for surveyors. GPS survey devices with microstrip antennas all experience some phase center variation, resulting in positional errors. The degree of unwanted variation of phase center is partly a function of the cross-polarization levels. Reducing phase center variation may be accomplished by reducing the cross-polarization levels or improving the circularity of survey antennas. The phase center of an antenna is located at the apparent center of curvature of the radiated equiphase surface for a given component of the far field radiation, assuming the equiphase surface is spherical or at least locally spherical.
So, there is a need to form a microstrip antenna with high quality circular polarization to reduce phase center variation. In fact, high quality circular polarization is a requirement for many satellite communication and sensor technologies. There are many forms of circularly polarized microstrip antennas. Some of the forms of circularly polarized microstrip antennas are circularly shaped or rectangular shaped patches. Circularly shaped or rectangularly shaped microstrip antennas can be fed by direct connection or through electromagnetic coupling. These circularly polarized microstrip antennas may be excited by a single feed or multiple feeds. Multiple feed antennas provide better circular polarization than single feed antennas when an appropriate offset in the feed excitation is used.